schist - greenschist facies of the Pelona schist - UNIT OF 5 STUDENT SPECIMENS
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This attractive grayish green schist is mapped as a greenstone schist, has an uneven foliated parting, is composed of chlorite, epidote and albite feldspar. Schists vary greatly in composition and appearance. They are often derived from clays and muds and shales and fit into a metamorphic sequence of slate, phyllite, schist and gneiss in order of increasing metamorphosis. They can also be derived from fine grained igneous rocks. This schist, a unit of the Pelona Schist, was metamorphosed from silica poor igneous rocks. The specimens were collected in San Antonio Canyon, San Gabriel Mountains, Los Angeles County, California.
Mica schists are the most common, with the mica making a schist easy to recognize. The Pelona Schist is quite variable. These specimens are distinctly green (for a rock - and not green like your lawn). Schists have at least 50% of the mineral grains in alignment. If less than 50%, the rock is a gneiss. This rock can be cleaved or split along layers, illustrating the origin of its name, from the Greek skhistos for split.
Schists are often described by their dominant mineral such as garnet schist or biotite schist. Chlorite schist is largely composed of chlorite - Greek chloros = green. Have your students compare this schist with the fine-grained gray schist we offer. In those specimens the mica flakes impart a silvery sheen. A good example of the variability of schists.
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